The invention relates to a data processing device for recovering digital data which is received in an analogue transmitted signal.
Digital data transmitted with an analogue television signal, such as Teletext, a VPS signal or a WSS signal, is corrupted by the transmission channel owing to its low-pass filtering effect, for example. The digital data is then reconstructed in a receiver by thresholding. The output of a conventional data processing device then provides a digital data stream which is supplied to a downstream decoder for error correction, interpretation of the data and grouping the bits into bytes.
A conventional data processing device using thresholding is only inadequately able to correct corruption caused by a transmission channel. Hence, the interpretation in the downstream decoder is incorrect or individual sections of data are not decoded, so that visible errors are shown on the screen. The data efficiency thus depends on the transmission channel.
The invention is based on the object of specifying a data processing device which guarantees a high data efficiency for the reception of digital data in an a analogue signal via transmission channels of various quality.
The invention achieves this object with the features specified in claim 1.
Advantageous developments are described in the subclaims.
According to the invention, the received signal is digitized and a threshold is determined in a threshold estimator. The interference in the channel is corrected in a downstream adaptive equalizer, and a bit clock generator produces the correct sampling rate and sampling phase for the data for a line.
The threshold estimator is used to determine the mean value of an input signal for each line, and this value is then taken as the optimum decision value for sampling the data bits for this line.
By using an adaptive equalizer, it is possible to correct different types of distortion, caused by the transmission channel, without the need for a training signal. The data processing device according to the invention can thus be used universally for all digital additional signals occurring in a television signal, for example.
The bit clock generator produces the correct sampling rate for recovering the data and adjusts itself to the optimum sampling phase.
The adaptive equalizer improves the data processing device by making it insensitive to channel interference. In addition, a data rate converter is preferably connected downstream of the adaptive equalizer, whose outputs have the correct data signals and the correct clock signal for decoding the data for a line.
To keep the mean value provided by the threshold estimator for a line constant, a clamping circuit may also be provided.
This clamping circuit can then have a scaling device connected downstream in order to provide the adaptive equalizer with a signal amplitude having a suitable dynamic range.
As a preference, it is additionally possible for a decision device to be used which releases only processible data for the adaptive equalizer. In addition, this decision device supplies the scaling device with a correct scaling factor and also checks the mean value of the threshold estimator before it is supplied to the clamping circuit.
A further embodiment of the invention involves the use of a black-level estimator which estimates the black level of every line, this black level then being used as a reference value for the threshold estimator and for the decision device.
To reduce the influence of interference signals, an input low-pass filter is preferably used.